At present, two-package solvent-based polyurethane coatings are widely used as coatings for substrates, such as metals, wood and plastics. These two-package solvent-base urethanes can be cured at room temperature or cured at relatively lower temperature. Such urethanes yield coatings with high levels of mar resistance and chemical resistance. They are so good that they often give more protection than is needed by the substrate. Because these coating compositions are made with organic solvents, which must be liberated into the atmosphere, they cause environmental problems which makes desirable a switch to non-toxic, e.g., aqueous-based compositions. Moreover, because the solvent-containing compositions are ofter reduced (i.e., thinned) with strong non-polar to medium polar solvents, they often attack and degrade plastic substrates to an undesirable degree. Non-polar thermoplastics, such as aromatic polycarbonates, e.g., of bisphenol-A and phosgene, or polyphenylene ethers, e.g., poly(2,6-dimethyl-1,4-phenylene)ethers, are capable of being dissolved and/or attacked by such non-polar solvents, and they can be distorted with excessive heat. From the foregoing it follows that ideal coating compositions should be water-based, fairly polar, and capable of curing at room temperature or only slightly higher temperatures.
A series of two-package waterborne polyurethanes meeting all three requirements has now been developed, and is the subject matter of this invention. The coating compositions are based on a prepolymerized polyurethane dispersed in water, the particles of the prepolymer having carboxyl and other ionic functional groups. These are further reacted with a trifunctional aziridine compound to yield a reactive composition which, when coated by conventional means on a substrate, produces a macro-polymeric coating with excellent chemical resistance and good durability. Good substrate protection can be achieved with these waterborne systems without the use of any organic solvent whatsoever. The coatings can also be formulated to include waterborne thermoplastic carboxylated acrylics, to reduce the amount of more expensive polyurethane, and without greatly affecting physical properties. In an essential aspect, to provide adhesion to non-polar plastic substrates, it is an essential feature of the invention to add to the composition a small, effective amount, e.g., 1-7% of N-methylpyrrolidone, Surprisingly, the plastic substrate is not at all degraded in so-doing.
Protective coatings for metals, bright or dull, are also needed. For example, bright metallized plastics, wherein small amounts of metal are vacuum sputtered or vacuum metallized over the plastic, now popular with the auto industry because of their light weight, need protective layers to prevent scratching and marring of the brilliant surface. Metal wheel covers (hub caps) also require protective coatings for their preservation and lasting beauty. In addition, protective coatings are widely used on the surfaces of rigid, formed thermoplastics.